There have recently been several proposals for high capacity distributed switching systems that make use of the very large bandwidth and high connectivity of passive optical star networks These systems use wavelength division multiplexing to broadcast many independent channels across the whole network. They offer capacities which are orders of magnitude larger than electronic (time multiplexed) networks, complete flexibility of interconnection configuration, service transparency and almost limitless capacity for future upgrades. These advantages are attractive for distributed switching of video signals, broad band overlay for fibre optic telecommunications to domestic subscribers, high capacity inter-exchange communication, high capacity switching networks for multiprocessor computers and ultra-high-speed telecommunications packet switching. Ultimately, multichannel coherent transmitters and receivers will allow thousands of independent gigaHertz bandwidth channels to be distributed and switched between thousands of nodes. Each independent channel uses a separate wavelength and is broadcast to all the nodes on the passive network. Switching is achieved by using either fixed wavelength transmitters and tunable receivers or vice versa. In either case, the major problem limiting the number of channels in such systems is the tolerance of the control of transmitter and receiver wavelengths with respect to the defined channels. For a large number of channels the transmitter and receiver components must both have very tight tolerance on operating wavelength and must operate over the widest possible wavelength range. Consideration of the development timescales of coherent system components and likely system requirements in the 1990's suggests that a simpler system based on direct detection with up to about 50 separate channels and a few hundred nodes would find widespread application.
This invention relates to lasers, one of whose potential uses is as the transmitters of such direct detection systems. These lasers are multicavity lasers in which an individual laser has a set of optical cavities optically in parallel and includes an optically dispersive element that is common to all cavities of the set. Preferably this dispersive element is a diffraction grating. The inclusion of a diffraction grating in a single optical cavity laser is already known for instance from the paper by J. Mellis et al entitled `Miniature Packaged External-Cavity Semiconductor Lasers for Coherent Communications`, 14th European Conference on Optical Communication IEE No 292 Part 1, Sep. 1988, pp219-222, but in that instance the grating is employed to form a dispersive reflecting element at one end of a single cavity, orientation of which provides tuning of its single waveband, and is organised so that its orientation may be adjusted to provide tuning of the laser emission. That device is thus essentially a tunable single channel device rather than a switchable multichannel device.
A multichannel device is described in patent application No. GB 2 204 404 A with n individual channels. In this device a reflector which is common to all n channels is formed at or near one end of an output fibre. This common reflector faces a diffraction grating and associated imaging optics which forms a diffracted image of the reflector along the line of a linear array of n semiconductor laser amplifiers. Reflection at the end facets of the laser amplifiers which face the diffraction grating is suppressed so that the facets at the opposite (distal) end, those remote from the diffraction grating, form a set of n discrete reflectors which co-operate with the common reflector in forming a set of n external reflector semiconductor laser cavities. A characteristic of this arrangement is that the optical gain is provided optically between the diffraction grating and the set of n external reflectors, and hence the power capable of being launched into the common output channel provided by the output fibre is limited by losses associated with the diffraction grating and its imaging optics.
Another form of multichannel device is described in U.S. Pat. No. 4,696,012 (Harshaw). This device has a mirror divided up into n discrete sections, reflectors, whose reflectivities are individually controllable. These n reflectors are optically coupled, via a diffraction grating, with a single reflector to define a set of n optical cavities for which the single reflector is common. The device has a single laser amplifier which is located between the diffraction grating and the common reflector. Specifically the laser amplifier is a molecular gas laser amplifier, but other forms of laser, including solid state lasers, are expressly contemplated.
The present invention is specifically concerned with multichannel cavity lasers in which optical gain is provided by semiconductor laser amplifiers. In such semiconductor multichannel cavity lasers the inclusion of a dispersive element such as a diffraction grating within the cavities typically means that the length of each optical cavity is at least 2 cm. In consequence the longitudinal modes of any individual cavity are very closely spaced in wavelength. Typically this wavelength separation is smaller than the resolution of the grating, and also smaller than the resolution provided by the practical size of individual controllable reflectivity discrete reflectors of a multichannel laser structure of the general configuration described above with reference to the disclosure of U.S. Pat. No. 4,696,012. In consequence, more than one longitudinal mode is liable to lie within the amplification waveband for any particular physical disposition of the particular discrete reflector, the diffraction grating and the laser amplifier that is common to all the channels There is thus the potential for multimode operation. In the case of a single channel external cavity laser, multimode operation can be suppressed by fine adjustment of the position of the external reflector, or of the orientation of the grating, in order to centre a particular one of the longitudinal modes at the peak of the gain curve, and thus obviate the possibility of having no mode at the peak, but instead two competing modes equidistant down on either side of the peak. This kind of mechanical adjustment approach is not possible in the case of a multichannel cavity laser in which the relative positions of all the discrete reflectors is fixed. Multimode operation of this sort is greatly to be deprecated in most information transmission systems because of the noise it engenders